The number of reported anti-Semitic incidents in the United States surged 57 percent in 2017, according to an annual report by the Anti-Defamation League.

The organization’s Audit of Anti-Semitic Incidents, released Tuesday, found 1,986 such incidents in 2017, compared with 1,267 in 2016. That increase was the largest in a single year since the A.D.L. began tracking in 1979.

Only once since 1979 has the Anti-Defamation League recorded more incidents: 2,066 in 1994. Since then, the numbers had mostly declined. There were small increases in 2014 and 2015. Then, in 2016, the count began to shoot up.

“It had been trending in the right direction for a long time,” Jonathan A. Greenblatt, chief executive of the A.D.L., said in an interview. “And then something changed.”

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Anti-Semitic incidents, long trending downward, increased 35 percent in 2016 and 57 percent in 2017.CreditAnti-Defamation League

That “something” is hard to identify definitively, but Mr. Greenblatt pointed to three likely factors: the increasingly divisive state of American politics, the emboldening of extremists, and the effects of social media. Some of the increase may also be attributable to better reporting of incidents.

The invigoration of the far right, including white supremacists and neo-Nazis, has been on display at events like a rally in Charlottesville, Va., in August that turned deadly when a man drove a car into a crowd of counterprotesters. A separate Anti-Defamation League report released last month found a more than 250 percent increase in white supremacist activity on college campuses in the current academic year. (The count released Tuesday does not include white supremacist incidents unless they had a specific anti-Semitic element.)

“The diminishment of civility in society creates an environment in which intolerance really can flourish,” Mr. Greenblatt said. And the platforms of social media, he added, have “allowed the kind of poison of prejudice to grow at a velocity and to expand in ways that really are unprecedented.”

The count by the A.D.L., an international organization that fights anti-Semitism and other forms of prejudice, includes three categories: harassment (1,015 incidents in 2017, up 41 percent from 2016), vandalism (952 incidents, up 86 percent) and assault (19 incidents, down 47 percent). The decrease in assaults was “the one piece of good news in this report,” Mr. Greenblatt said.

For the first time in at least a decade, incidents were reported in all 50 states. And, unusually, K-12 schools had more reports than any other location. (Typically, public areas have the most.) Incidents at those schools nearly doubled, to 457 from 235; those on college campuses increased 89 percent, to 204 from 108.

Many of the incidents involved swastikas etched on school property or drawn on Jewish students’ belongings.

The increase in expressions of anti-Semitism among students is “astounding” in its size, Mr. Greenblatt said, but also not entirely surprising.

“Kids repeat what they hear,” he said. “And so in an environment in which prejudice isn’t called out by public figures, figures of authority, we shouldn’t be surprised when we see young people repeat these same kind of tropes.”

The count is based on reports from victims, law enforcement and the news media. The Anti-Defamation League’s 26 field offices in the United States often receive reports directly from victims or their loved ones. Other times, employees will see a post on social media and follow up with the poster.

In each case, the group confirms the information independently and assesses its credibility. Reports deemed not credible are not included in the tally.